The present invention relates generally to ion-exchange chromatography and, more particularly, to indirect photometric sensing of optically transparent ion concentrations by frontal analysis.
The determination of which inorganic and/or organic ions are present in solution is a common place and classical analytical problem. One of the most common methods of determining which inorganic and/or organic ions are present in a concentration is by chromatography. The determination of inorganic and/or organic ions which are strong ultraviolet (UV) absorbers in solution has presented few problems to the ion chromatographer. However, the analysis of inorganic and/or organic ions which are weak UV-absorbers or non-UV absorbers in solution has been problematic.
One approach to solving the detection of weak or non-UV absorbers problem is by ion chromatography-eluent suppression. This method is outlined in U.S. Pat. No. 4,474,664, issued to Stevens et al and assigned to the same assignee as the present application, the specification of which is herein expressly incorporated by reference.
Another approach to detect weak or non-UV absorbers is by indirect photometric chromatography (IPC). Briefly, IPC requires the addition of a UV absorbing ion to the mobile phase. A sample plug of unknown ions is injected into the mobile phase. The transparent or non-UV absorbing ions elute through an exchange column and are detected as transients. The detector is coupled with a recorder to provide a chromatogram chart of the results. The addition of the UV absorbing ions to the eluent causes an elevated baseline on the chart. As the non-UV absorbing ions are eluted, negative peaks are produced on the chart corresponding to the qualitative and quantitative information of the sample ions. This method is illustrated by U.S. Pat. No. 4,414,842, issued to Small et al and assigned to the same assignee as the present application, the specification of which is herein expressly incorporated by reference.
Another method of determining ion concentration is by ion selective electrode (ISE) systems. These devices are used especially in clinical environments. ISE systems are among the most widely used chemical systems presently available. Unfortunately, ISE systems are not truly specific but respond more or less to a variety of interfering ions. Other problems associated with the ISE systems are long term drift and accuracy in maintenance requirements. These conditions require that a technically experienced person operates and maintains the apparatus.
While ion chromatography is a reliable method for determining ion concentrations, in its present form, it requires costly equipment and technical expertise to run and maintain this equipment. Thus, a need for inexpensive, simple and reliable methods for ion determination is present in the field.
Thus, there is a desire in the art to move toward portable or miniature apparatus which will enable the determination of ion concentrations away from the laboratory. Also, the procedures should be simple and capable of being interpreted by nontechnical personnel. While the analytical instrumentation should be small, simple and inexpensive, it should also be highly reliable and self-sufficient.